Rising life expectancy is contributing to rapid increases in the size of the older population and is expected to lead to a sharp rise in dementia cases, from about 47 million people worldwide today, to potentially more than 140 million in 2050. In this project, we propose to advance our understanding of protective and risk factors for late-life cognitive decline and Alzheimer's disease and Alzheimer?s disease-related dementias by taking advantage of new harmonized data from the HRS family of surveys across 33 countries. We propose a novel life-course model of risk factors, building on the recently proposed model by the Lancet commission, which links the risk of Alzheimer's disease and Alzheimer?s disease-related dementia to early and mid-life factors, within the context of historical, institutional, and physical environments. This proposal seeks to explore how psychological well-being and the risk of Alzheimer's disease and Alzheimer?s disease-related dementias evolve later in life and how these are affected by major life changes, such as retirement, and changes in family circumstances (for instance death or major health events of a spouse). In doing so, we place special emphasis on the role of early life conditions, both as determinants of observed cognition and psychological wellbeing and as moderating factors in the explanation of the effects of major life changes later in life. We will be analyzing rich and comparable data from 31 countries, by taking advantage of the fact that a number of the main aging studies in the Western world (Health and Retirement Study, HRS; English Longitudinal Study of Aging, ELSA; Survey of Health, Ageing and Retirement in Europe, SHARE) have recently included (or are about to include) life history modules eliciting information about childhood health, family circumstances, education, and residential mobility. In addition to these self-reports we will make use of contextual and institutional information, both at present and at the time of a respondent?s childhood (such as the state of the business cycle or infant mortality). The contextual information will help in identifying causal effects of circumstances and events during a respondent?s childhood and adult life on later life cognitive function and risk of Alzheimer's disease and Alzheimer?s disease-related dementias, and psychological well-being.